The invention relates to cable television systems, and more particularly to such systems capable of providing interactive services provided over a cable television network.
Bandwidth problems have long restricted the ability of cable television systems to provide information services to subscribers. Although a coaxial cable system may permit a cable system operator to provide, for example, 50 television channels, each 6 MHZ wide, with a total bandwidth of 300 MHZ, this total bandwidth is insufficient to permit an arrangement wherein each subscriber may have, in addition to these 50 channels, an interactive information service that functions independently of interactive information services to all other subscribers and provides full color video, motion typical of movies or television, and sound.
The reason for the insufficiency in bandwidth is apparent on a consideration of the demands on the system. Typically a subscriber on a cable system obtains information services over a communication path that starts at the head end, proceeds over one of typically a number of trunks, and then over one of a number of feeders, and then over one of a number of taps. Each feeder may have, for example, fifty or more subscribers, and each trunk might serve a hundred or more feeders. The result is that 5000 subscribers per trunk is not atypical. Thus merely to provide a private one-way information service, and nothing else, to each of these 5000 subscribers would require the trunk to carry 5000 different signals, each using about 6 MHZ of bandwidth, and would alone require a trunk bandwidth of 30 GHz, which is nearly two orders of magnitude greater than provided by a typical coaxial cable system.
The use of fiber optic trunks can assist in providing additional bandwidth. but to the extent that coaxial cable secondary trunks and feeders are used in a hybrid fiber-cable system, bandwidth limitations may continue to pose problems. While video compression schemes may assist in bringing the bandwidth requirements within more practical limits, each subscriber would then need to be provided with his own decompression unit.
Another problem lies in how to handle the switching and computing demands on the head end to provide separate and private information service to potentially hundreds of thousands of subscribers simultaneously.
One approach to the above problem involves placing a digital computer in the home of each subscriber to provide interactive service, where the computer operates over a traditional network of computers. However, with the above approach, a large capital investment is needed to provide each subscriber with a computer.